The flow passing under a ground vehicle imparts a drag force to the vehicle when it impinges on and flows around the vehicle undercarriage components, landing gear, axels, brake components, mud flap systems, wheel wells and fenders, wheels, tires and various other vehicle components attached to or a part of the underside of a vehicle. The ground vehicle class of particular interest is a tractor-trailer truck system consisting of a motorized lead vehicle pulling one or more non-motorized vehicles. The present invention is designed to control the flow from entering the undercarriage region from the side of a trailer of a tractor-trailer truck system.
There have been several attempts to reduce the aerodynamic drag associated with the undercarriage of the trailer of a tractor-trailer truck system. Trailer undercarriage drag may comprise 25 percent of the total vehicle drag.
The trailer undercarriage is comprised of all the components located below the trailer floor deck surface and the ground or road surface and includes all components attached to the trailer in this region. The flow passing around the tractor-trailer truck vehicle enters the undercarriage region from the trailer side and from the tractor undercarriage region. The undercarriage flow of a trailer is characterized as unsteady and dynamic and comprised of various size and strength eddy currents. The unsteady nature of the undercarriage flow is a result of the flow interacting with the ground or road, rotating wheels, brake systems, axels, tractor undercarriage flow, and the various components comprising the trailer lower surface. Relative to the free stream static pressure, the undercarriage flow imparts an increased pressure on surfaces that face forward and a decreased pressure on surfaces that face aft. The increase in pressure acting on the forward-facing surfaces and the decreased pressure acting on the aft-facing surfaces both generate an aerodynamic drag force. It is estimated that the pressures acting on the trailer wheel assembly accounts for one-half of the undercarriage drag, with the remaining drag being attributed to the flow interacting with numerous small structures comprising the trailer undercarriage. Previous attempts have addressed the undercarriage drag by installing either spanwise or streamwise aerodynamic fairings to the trailer underside to either divert undercarriage flow from the trailer wheel assembly or to block flow from entering the undercarriage region from the trailer side. The flow diverter devices are spanwise fairings that mount to the trailer undercarriage immediately forward of the trailer wheel assembly. The flow diverter fairings are angled downward or outward to divert the undercarriage flow from the wheel assembly. The flow blocking devices are streamwise fairings that mount beneath the trailer outside edge between the trailer wheel assembly and the tractor rear wheel longitudinal position, when the tractor and trailer are joined. Both types of fairings show increased benefit with increased vertical extent of the fairing.
Conventional approaches have used the flow diverter undercarriage fairings to reduce the mass of undercarriage flow that impinges onto the trailer wheel assembly, as shown in U.S. Pat. Nos. 4,386,801, 4,486,046, and 4,640,541. These representative fairing devices, while successful in reducing the mass of flow impinging on the trailer wheel assembly and thereby reducing the wheel assembly drag, do not significantly affect the trailer undercarriage drag. The limited effectiveness of these devices is a result of the drag generated by the device, referred to as device drag. The device drag for these fairings may be equal to the wheel assembly drag. These devices only reduce the wheel assembly drag and do not reduce the remaining undercarriage drag associated with the various trailer components.
Other approaches have used the trailer undercarriage side fairings to reduce the mass and velocity of the flow entering the undercarriage region of a trailer, as shown in U.S. Pat. Nos. 4,611,847, 4,746,160, 5,280,990, 5,921,617, and 6,644,720. These representative flow blocking devices, while successful in reducing the mass of flow entering the trailer undercarriage region are either simple rigid structures or they are complex active, flexible and variable geometry systems. The simple devices are designed to have a limited vertical and longitudinal extent in order to reduce the impact on operations and maintenance. Limiting the vertical and longitudinal extent of the device significantly reduces the flow blocking capability and results in a minimal aerodynamic drag reduction benefit. The complex devices typically have features that are active, flexible, and/or variable in order to maximize the flow blocking capability while minimizing the impact on operations. The complex devices typically consist of multiple components. The complexity of these devices results in increased weight, maintenance, and cost. Each of the trailer undercarriage flow blocking devices consists of a vertically extended structure that attaches to the trailer lower surface outer side edges. These devices are held in position by various support and bracing structures that are integrated into or attached to the inward facing surface of the flow blocking structure. The support and bracing structures add additional forward and rearward facing undercarriage elements that contribute to the undercarriage drag. These support and bracing structures also collect debris, snow and ice during operation resulting in an increase in maintenance and repair requirements.